The present invention relates to an antilock brake control device for efficiently braking the wheels of an automobile, particularly the rear wheels of the automobile.
In order to brake the wheels of an automobile with maximum efficiency, the braking pressure must be controlled so as to follow changes in the coefficient of friction between the road surface and the wheels. For this purpose the brake assembly should be provided with an antilock device which is adapted to increase, hold and reduce the braking pressure so as to repeatedly and alternately release and apply the brake at very short time intervals even while the brake pedal is being trodden.
Such an antilock control device includes wheel speed sensors for detecting the rotation of the wheels, an electronic control circuit for calculating the wheel speeds, estimated vehicle speed (reference wheel speed) and slip rates and for producing control signals. Such control signals include pressure signals, such as pressure reduction, pressure hold and pressure increase signals, to be applied to a fluid pressure control unit which controls the braking pressure generated in a master cylinder and applied to the wheel cylinder.
More specifically, the electronic control circuit performs various arithmetic operations based upon the wheel rotation signals to determine the slip rate and deceleration, judges from the fluctuation of the slip rates and deceleration whether each wheel is falling into a locked state or recovering from the locked state, and provides control signals such as pressure reduction, pressure hold and pressure increase signals on the basis of this judgement.
A typical prior art fluid pressure control unit includes solenoid valves (optionally provided with cutoff valves or flow control valves), check valves, hydraulic pumps with motors, accumulators and reservoir tanks. The valves of one of the abovementioned types are provided in the brake pressure lines between the master cylinder and the respective wheel cylinders to control the flow of braking pressure or pump pressure.
As a method for controlling the flow of braking pressure to the wheel cylinders, it is known to provide one control unit for each vehicle wheel to control the wheels independently of one another (four-channel mode), to provide one control unit for each of the right and left front wheels and one for both rear wheels (three-channel mode), and to provide one control unit for each of the right and left front wheels and adapt the system so that the hydraulic pressure acting on each of the rear wheels follows the hydraulic pressure acting on one of the front wheels (2-channel mode).
As a method of providing fluid pressure control signals to the above-described hydraulic circuit, it is known to control the two wheels at each side as one unit and reduce the braking pressure on all the wheels if the wheels at one side, which are controlled with a lower fluid pressure (which means that the coefficient of friction with the road surface is smaller), show a tendency to lock (such a control method is hereinafter referred to as the select-low mode). Another known method is to reduce the braking pressure on all the wheels if the wheels at one side, which are controlled with a higher fluid pressure, show a tendency to lock (select-high mode). It is also known to control the braking pressure on the wheels independently of one another according to the road condition (independent mode).
The select-low mode is effective in increasing the resistance of the wheels to lateral forces and thus in improving the directional stability and the drivability of the vehicle. However the select-low mode has a problem in that the braking distance tends to be long because of an insufficient braking force. On the other hand, if the wheels are controlled in the select-high mode, the vehicle can be braked with a sufficient braking force, but its directional stability will be insufficient. With the independent mode, the wheels can be controlled individually so as to precisely conform to the changing road condition. In view of the advantages and disadvantages of each control mode, it is desirable to control the front wheels on the independent or select-high mode to assure a sufficient braking force and to control the rear wheels on the select-low mode to maintain a high directional stability.
However, with the abovementioned prior art antilock brake control devices, either all the wheels are always controlled independently of one another, or the rear wheels are always controlled in the select-low mode irrespective of the road condition.
Suppose now that the rear wheels are always controlled in the select-low mode. Since it is practically impossible to eliminate the manufacturing errors for the fluid pressure control unit, the braking pressures on right and left wheels can increase and decrease in different manners from each other, thus bringing about differece in braking pressure between right and left wheels, in spite of the fact that the same control signals are provided to them. Even if the difference in pressure is small at first, it will gradually increase because the rear wheels are always controlled in the select-low mode. It will be appreciated from FIG. 7 that the distance between the curves b and a and the distance between the curve b' and a' increase with time (wherein the curves b and b' indicate the wheel speed and the braking pressure, respectively, if the pressures on both wheels are to increase and decrease in exact conformity with each other, whereas the curves a and a' indicate the actual wheel speed and the braking pressure, respectively). This means that the braking force on one of the rear wheels at the low pressure side is not effectively utilized.
With the independent control mode, where all the wheels are controlled independently of one another, if the difference in the coefficient of friction of the road surface between right and left sides of the vehicle is large and if the sum of slip rates of both rear wheels exceeds a predetermined value, the difference between the braking forces applied to both rear wheels will increase excessively. This will bring about a lag in the brake timing between both rear wheels and a difference in the pressure increase and pressure reduction intervals between both rear wheels as shown by curves c, c', d and d' in FIG. 8, thus hampering the directional stability of the vehicle (the vehicle tends to lose its stability in the areas A and B). In such a case, the rear wheels should instead be controlled in the select-low mode in order to improve the directional stability .
However, it is not desirable to always control the rear wheels in the select-low mode. If the sum of slip rates of both rear wheels falls below a predetermined value, the difference between the braking forces applied to both rear wheels is so small as to not hamper the directional stability of the vehicle. Controlling the rear wheels on the select-low mode in such a situation would unnecessarily reduce the braking force on the rear wheels, thus increasing the braking distance.